Both mental stress and platelet activation are suspected triggers of cardiovascular events like unstable angina or myocardial infarction. We wanted to study the effect of mental stress on platelet activation in patients with coronary artery disease and compare it to patients without coronary artery disease. With a technique called impedance aggregometry, we measured platelet function on blood collected upstream and downstream from the coronary circulation. The study took place in the catheterization laboratory. We measured platelet function at rest and in response to speeding up the heart rate to mimic exercise and also after a three minute period of mental stress. To cause mental stress we asked the patient to play a video game during their catheterization. We have shown in the past that, only in patients with significant narrowing in their coronary arteries, platelets respond to speeding up of the heart rate by increasing their tendency to form clots, and this only in the coronary circulation. In this study we confirmed that result and also showed that mental stress causes a more generalized activation of the platelets, but again more so in the patient with coronary artery disease. In conclusion, it is possible that mental stress is a trigger of cardiovascular events by causing an increase in platelet activity.